As a general internal gear pump that is known widely, there is a trochoidal pump utilizing a trochoidal tooth profile for an inner rotor and an outer rotor. In the trochoidal pump, as the inner rotor is rotationally driven, the outer rotor which meshes with the inner rotor is rotated in the same direction as the inner rotor. This rotation increases and decreases the volume of a pump chamber formed between contact parts of the rotors, thereby suctioning a fluid from a suction port and discharging the fluid from a discharge port. Since this trochoidal pump has advantages, such as good efficiency and ease of fabrication, it has come into wide use.
The internal gear pump as described above is used as an oil pump of a prime mover, and the inner rotor is rotationally driven by using a crankshaft of the prime mover as the driving shaft (for example, Japanese Unexamined Patent Application, First Publication No. H11-343985 (FIG. 8, Paragraph 0019)).
An example of the internal gear pump will now be described with reference to FIG. 14. Specifically, an internal gear pump 1 is assembled such that an inner rotor 4 is inscribed to an outer rotor 3 in an eccentric state in a rotor chamber 12A of a casing 2. The outer rotor 3 has internal teeth 3A formed in the shape of circular arc teeth at an inner periphery thereof, while the inner rotor 4 has external teeth 4A formed in the shape of trochoidal teeth at an outer periphery thereof. These outer and inner rotors mesh with each other while forming a plurality of voids. The number of the external teeth 4A of the inner rotor 4 is one less than the number of the internal teeth 3A. Also, the outer rotor 3 is rotatably fitted into the rotor chamber 12A of the casing 2. Moreover, the inner rotor 4 has a mounting hole 5 in the central axis thereof, and a crankshaft 6 that is a driving shaft is inserted into and connected to the mounting hole 5. Furthermore, a suction port 7 and a discharge port 8 are formed in the rotor chamber 12A of the casing 2 with the central axes of both the rotors 3 and 4 therebetween. When the internal gear pump is used, the inner rotor 4 rotates via the crankshaft 6. With the rotation of the inner rotor, the outer rotor 3 also rotates in the same direction by engagement between the internal teeth 3A and the external teeth 4A. While the outer rotor 3 and the inner rotor 4 make one rotation, the volume of each void part increases or decreases whereby oil is suctioned in the suction port 7, and oil is discharged from the discharge port 8.
Also, in the internal gear pump 1 in which the inner rotor 4 is rotated by the crankshaft 6 of an engine, in order for the crankshaft 6 to be inserted into and connected to the mounting hole 5 of the inner rotor 4 after the outer rotor 3 and the inner rotor 4 are assembled into the casing 2, a clearance that enables insertion is provided between the mounting hole 5 and the crankshaft 6 so that centering of the central axis of the inner rotor 4 can be obtained by engagement with the casing 2.
As the above engaging structure, for example, an axially projecting tubular part is provided at a side face of an inner rotor, a supporting hole which supports the tubular part is provided in a casing (for example, Japanese Unexamined Patent Application, First Publication No. S63-223382 (first line from the bottom in the lower right column of Page 2 to first line in the upper left column of Page 3, and FIGS. 5, 6, and 8)), and the supporting hole defines the center of rotation of the inner rotor. In this case, the clearance between the tubular part and the supporting hole is set smaller than the clearance between the mounting hole and the crankshaft.
In the structure in which a predetermined clearance is provided between the mounting hole and the crankshaft as described above, in order to positively transmit rotation of the crankshaft to the mounting hole, a pair of flat surfaces are formed at the outer periphery of the crankshaft (for example, Japanese Unexamined Patent Application, First Publication No. H11-343985 (FIG. 8, Paragraph 0019)), Japanese Unexamined Patent Application, First Publication No. S63-223382 (first line from the bottom in the lower right column of Page 2 to first line in the upper left column of Page 3, and FIGS. 5, 6, and 8).
When the above engaging structure between the crankshaft and a mounting hole is shown in FIGS. 15 and 16, the flat surfaces 6A and 6A are formed at the outer periphery of the crankshaft 6, the mounting hole 5 which the crankshaft 6 is inserted into and connected to is formed substantially in substantially the same shape, and predetermined clearances C are provided between the flat surfaces 6A of the crankshaft 6 and the mounting hole 5. In addition, in FIGS. 15 and 16, the clearances C are shown larger than the actual dimensions for the purpose of explanation. Accordingly, in the structure shown in FIGS. 15 and 16, rotational moment is transmitted to the mounting hole 5 at two corners 6B located on one side of the flat surfaces 6A of the crankshaft 6 in the direction of rotation thereof. For this reason, since stress is concentrated in the vicinity of corners 5A of the mounting hole 5, deterioration of durability is caused, and high surface pressure is generated in a transmission part, abnormal noises are apt to be generated. Also, when a sintered part is used for the inner rotor, it is necessary to secure the strength of the whole inner rotor in accordance to the maximum stress.
Moreover, since the corners 6B that are edges of the crankshaft 6 strike against the mounting hole 5, there is a problem in that the mounting hole 5 is worn out in the portion against which the corner 6B hits. Furthermore, if hard foreign objects enter the clearance between the mounting hole 5 and the crankshaft 6, the mounting hole 5 is damaged easily.
It is an object of the present invention to provide an inner rotor of an internal gear pump capable of relaxing any local stress concentration caused by a rotational moment transmitted from a driving shaft.